The assessment of optical hazards in recent years has led to the recognition of the possible hazards to the retina associated with blue light (400-500 nm). If the blue light hazard is a real threat to vision, then the UV/visible transmission characteristics of ophthalmic lenses, and intraocular lenses (IOLs) in particular, should be modified to provide adequate protection from blue light hazards encountered in the environment.
In the ambient environment solar radiation is the primary hazard to vision. The sun freely emits UV, visible and IR radiation much of which is absorbed by the atmosphere. The solar radiation that is transmitted through the atmosphere and reaches the earth's surface consists of UV-B radiation (230-300 nm), near UV or UV-A radiation (300-400 nm), visible light (400-700 nm) and near IR radiation (700-1400 nm). The ocular media of man in its normal, healthy state freely transmits near IR and most of the visible spectrum to the retina, but UV-B radiation is absorbed by the cornea and does not reach the retina. The transmission of near UV and the blue portion of the visible spectrum can be absorbed by the crystalline lens depending on age.
The human crystalline lens changes its UV and visible transmission characteristics as it ages. In infancy the human lens will freely transmit near UV and visible light above 300 nm, but with further aging the action of UV radiation from the environment causes the production of yellow pigments, fluorogens, within the lens. By age 54 the lens will not transmit light below 400 nm and the transmission of light between 400 and 500 nm is greatly diminished. As the lens ages it continuously develops a yellow color, increasing its capacity to filter out near UV and blue light. Therefore, after cataract removal the natural protection provided by the aged human lens is also removed. If the cataract is replaced by an IOL, usually UV protection is provided, but blue light protection is still lacking.
The use of conventional yellow dyes, such as commercially available 4-phenylazophenol (Solvent Yellow 7), 2-(2'-methyl)-phenylazo-4-methyl phenol (Solvent Yellow 12) and N-(4-phenylazo)phenyl diethanol amine (Solvent Yellow 58), in IOLs to block blue light is not desirable because these dyes are not bound to the lens material and thus may leach out of the IOL after it is inserted in the eye. These nonbonded dyes also cause problems in the manufacture of polymer lenses that are extracted with a solvent after they are formed. In this extraction step, the solvent may remove up to 90% of the non-bonded dye from the lens.
Japanese Kokai Patent Application No. Hei 1[11989]-299,560 ("Menikon Application") claims an intraocular lens material characterized by a polymerizable ultraviolet light absorber having a polymerizable group selected from an acrylol group, a methacrylol group, a vinyl group, an allyl group, and an isopropenyl group, and a polymerizable dye having a polymerizable group selected from an acryloyl group, an allyl group, and an isopropenyl group, which are copolymerized with other polymerizable lens-forming monomer components. Also taught by the Menikon Application are polymerizable dyes having a polymerizable group selected from methacryloyl groups, vinyl groups, and acryl groups. The Menikon Application lists numerous formulas representing hundreds of dyes. As examples of the polymerizable dyes of the azo system, the Menikon Application lists those of the general formula: ##STR1## where X.sub.17 may be, among others, any of the groups represented by: ##STR2## R.sup.1 is --H or --CH.sub.3 ; R.sub.23 may be, among others, --H, --OH, or a halogen atom;
R.sub.24 may not be H, but may be --OH, --CH.sub.3, --C.sub.2 H.sub.5, --OCH.sub.3, --OC.sub.2 H.sub.5, and halogen atoms; PA1 k, m, l, and n are integers of 0 or 1; PA1 R.sub.25 may be, among others, a benzene derivative substituted with C.sub.1 -C.sub.8 alkyl groups; PA1 R.sub.26 is --H, or C.sub.1 to C.sub.3 lower alkyl; and PA1 Y.sub.11 and Y.sub.12 are --NH-- or --O--.
The azo dyes taught in the Menikon Application suffer the following disadvantages, however. Directly attaching reactive acrylic/methacrylic groups or other electron-withdrawing groups, such as carbonyl, carboxylic acid, or ester groups, to the dye moiety weakens dye strength and may change dye color.
The effect of electron-withdrawing groups on the color and relative strength of a yellow dye can be quite pronounced. For example, the yellow dye known as Solvent Yellow 58 is converted into a red dye, Pigment Red 100, solely by the addition of a carboxylic acid group directly bonded to the phenylazophenyl dye moiety. ##STR3##
There is only one case in which the Menikon Application allows an acrylic/methacrylic group not directly bound to the azo dye moiety by an electron-withdrawing group. This case requires instead that an amino group be directly attached to the dye moiety. Even though amino azo dyes are useful, they are less desirable than phenolic azo dyes because the amino group accelerates the decomposition of peroxide initiators, such as those used in conventional free-radical polymerization processes.
Another example of dyes based on the amino azo system are the polymeric colorants based on acrylated chromophores of the type ##STR4## wherein R=CH.sub.3 or H; and the Ar group is phenyl, naphthyl, etc. Guthrie, "Polymeric Colorants," Rev. Prog. Color Relat. Topics, Vol. 20, 40-52 (1990). Substituents may be added to the aromatic groups to provide variations in color and other physical properties. The works of various people am summarized in this review article. Some of the work reviewed includes reactive azo dyes containing methacrylate, acrylate, epoxide and vinyl ester functionalities in the following applications and studies: optical recording materials, the non-linear optical susceptibility of copolymers containing acrylic azo monomers and methyl methacrylate, and the determination of copolymerization parameters and reactivity ratios for the copolymerization of azo dye monomers containing a methacryloyl functionality with styrene and with methyl methacrylate.
What is needed are additional polymerizable yellow dyes which are easily synthesized from commercially available dyes or other starting materials and which, when incorporated in ophthalmic lenses, will not be extracted out of the lens during solvent extraction or leach out of the lens after insertion in the eye.